Low distortion phototube



April 1, 1969 R. CLAYTON LOW DISTORTION PHOTOTUBE Sheet Filed Dec. 16, 1966 M k m5 W J PD v wm mA l 9\ 4 2 T. 7 Q Q 2 3 4 E v w 7 9 W l 6 m I. 0 Jill H P B CATH ODE INVENTOR H. CLAYTON ROBERT BY fi' fl ATTORNEYS April 1969 R. H. CLAYTON 3,436,551

LOW DISTORTION PHOTOTUBE Filed Dec. 16, 1966 Sheet 2 of EIE E ATTORNEYS United States Patent 3,436,551 LOW DISTORTION PHOTOTUBE Robert H. Clayton, Fayetteville, N.Y., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Delaware Filed Dec. 16, 1966, Ser. No. 602,328 Int. Cl. H013 39/12 U.S. Cl. 250-213 Claims ABSTRACT OF THE DISCLOSURE The present invention relates generally to phototubes and more particularly to a low distortion phototube. Phototubes of the type with which this invention is primarily concerned include in the usual instance a photocathode of extended area for receiving a radiation image thereon, electron lens means for focusing the electron image emitted by the cathode into an image plane, and a utilization electrode which may be a phosphor screen, or an analyzing aperture of an image dissector or the like. Initial electron emission from the surface of such a photocathode includes random velocity and directional components which can and do introduce aberrative and distortional effects in the output image.

Considering that the photocathode emits an electron image in response to a radiation image incident thereon, the electron-focusing system, usually in the form of an electrostatic lens, forms the electron image into a beam which converges to a cross-over and thereafter diverges to a focal plane. Such a beam can be considered as being composed of a multiplicity of rays or pencil-like bundles composed of electrons emitted by incremental, point-like spots on the photocathode. When it is recognized that the electrons initially emitted by the photocathode have both random directional and velocity components, it will be recalled that each individual ray or bundle of electrons initially follows a divergent path. It is necessary that this divergence be attended by a reconverging action in the lens system so as to reproduce with maximum fidelity the radiation image applied to the photocathode. Improvement in resolution can usually be achieved by reducing the magnification ratio of the lens system. However, it is usually true that the lens system provides convergence of the incremental rays or bundles in a different plane from the focal plane of the total beam such that an effort to reduce magnification almost always results in increasing the spot size of the image rays or bundles in the focal plane. This results in deterioration of the resolution. It will thus appear that in conventional electrostatically focused phototubes there have been two conflicting requirements in that changes in the magnification ratio carries with it changes in the convergence points of individual rays or bundles and vice versa. It is therefore desirable to provide for a change in the magnification ratio which is entirely independent and unrelated to spot resolution. It can, however, occur that convergence of the incremental rays or bundles controlled independently of the focusing of the entire beam will not result in a geometrically faithful reproduction of a focused image in a fiat plane or in a plane of the same shape as that of the photocathode. However, by shaping the paths of the indi- Patented Apr. 1, 1969 vidual rays or bundles identically on opposite sides of the beam cross-over, the reproduced image can be made to be an identical counterpart of the image on the photocathode.

It is accordingly an object of this invention to provide a low distortion phototube.

It is another object of this invention to provide a phototube in which magnification, electron bundle reconvergence, and correction for geometric distortion may be controlled independently.

It is still another object of this invention to provide an electrostatic focusing system wherein the electrostatic fields are bilaterally symmetrical about a plane perpendicular to the beam axis at the cross-over, this symmetry resulting in self-correction of each portion of the image with respect to geometric fidelity.

In accordance with the broader aspects of this invention, a low distortion phototube is provided comprising an element capable of emitting an electron image, this element being either a photocathode or a charge-storage electrode capable of modulating a beam to form an electron image. Means are provided for forming the electron image into a beam and focusing this image in inverted form into a plane spaced a predetermined distance from the electronemitting element. An electron-collecting device located in the image plane is utilized for providing a representation corresponding to the inverted image. The focusing means includes first lens means adjacent to the electron-emitting element for converging the total beam to a cross-over point disposed between the element and the aforesaid device. The beam diverges from said cross-over point, and second lens means are positioned adjacent to this cross-over point for reconverging electrons of the beam which have initial divergent trajectories. Lastly, third lens means as part of the focusing means are provided adjacent to the utilization device for exerting a converging force on the diverging portion of said beam such that it is possible to shape the diverging portion of the beam to correct for geometric distortions.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram showing beam and bundle focusing in conventional prior image tubes and is used in explaining the invention;

FIGS. 2, 3 and 4 are similar diagrams of electron optical systems used in explaining the principles of this invention; and

FIG. 5 is a longitudinal sectional view of a phototube as one embodiment of this invention.

Referring first to FIG. 1, a photocathode 1 lying in a fiat plane is shown as emitting electrons in response to radiation, these electrons being accelerated and formed into a beam shown schematically at 2. This beam is formed by an electrostatic focusing field forming an electrostatic lens schematically indicated by the numeral 4, this lens converging the beam to a cross-over point 3. The total beam 2 diverges after the cross-over point 3 and is imaged into a focal plane indicated by the numeral 5.

Considering now a single incremental illuminated pointlike spot 6 on the surface of the photocathode 1, the electrons emitted from the spot 6 can be considered to be a ray or bundle of electrons 7, all of the various rays or bundles of electrons emanating from the cathode 1 forming the total electron beam 2. The bundle 7 is shown in FIG. 1 by the double cross-hatching and its counterpart 9 emanating from another spot 8 on the cathode is also indicated by similar hatching. Electrons emitted from the spots 6 and 8 on the cathode have directional diversity such that each bundle 7 and 9 initially follows a diverging path until the lens 4 is entered. conventionally, the lens 4 reconverges these bundles 7 and 9 into a point-like spot in the focal plane 5. If the total beam 2 is also focused in this same plane 5 by the lens 4, it will thus appear that the image in the plane 5 will be of maximum resolution. It will thus be observed that the cross-over point 3 of the beam 2 does not occur at the same location or in the same plane as the point of convergence of the individual bundles 7 and 9, both of these conditions establishing optimum focus in the image plane 5. It will also be seen that the portion of the beam 2 which is focused in the plane 5 covers a larger area 10 than the counterpart area 11 of the cathode from which the beam was initially emitted. It will thus be seen that by merely moving the image surface 5 toward the photocathode 1, the magnification will be reduced, but convergence of the incremental bundles will not be completed. Instead of the relocated image surface 5 receiving point convergence of the individual bundles, it will be intersected by the larger crosssection of the bundles, thereby introducing a condition of deteriorated focus. Thus, unless the bundle convergence and beam focus can be individually adjusted, it will be seen that adjustment of only one will cause a corresponding adjustment in the other leading to reduced image resolution. The present invention provides means for adjusting one independently of the other.

As already explained in connection with FIG. 1, the beam 2 represented by the cross-hatched area provides a magnified image 10 of excellent resolution. For many applications, this large linear magnification limits the useful area of the cathode to that which may be imaged on the surface 5. In other words, only a small area 11 of the cathode 1 is used in contrast with the area 10 on the surface. If it is desired to use a larger area of the cathode 1, this can be accommodated by making available a larger area on the surface 5. This condition is indicated in FIG. 1 by the dashed line beam 12, the area of this beam originating at the cathode being indicated by the numeral 13 and the area of the reproduced image on the surface 5 being indicated by the numeral 14. For the beam 2, it will be noticed that it has a divergence angle For the beam 12, however, this angle increases to the value 6 shown in FIG. 1. This increase in divergence angle results in severe increases in distortion and field curvature of the image surface 5.

For these reasons, it is desirable to weaken the imagefocusing lens to provide more nearly unity magnification. As a consequence of this weakening, the divergence angle can be kept small and the field curvature flatter. Additionally, spherical aberration may be reduced as a third power function of the focal length of the lens.

The result of weakening the lens is graphically illustrated in FIG. 2, wherein unity magnification is provided by the lens 15. The beam 16, having a cross-over 17, is directed forward but unfocused at the plane 5. It will be noticed that the initially divergent electron bundles 7 and 9 as they intersect the image plane are as yet unconverged such that the image in the plane 5 is underfocused.

Now referring to FIG. 3, if another, relatively weak positive lens 18 is positioned at the cross-over 17, the individual bundles 7 and 9 still diverging as they leave the lens 15 can be reconverged onto the image surface 5. By adjusting the power of this lens 18, the exact point of convergence of the individual bundles 7 and 9 may be controlled to coincide with the plane 5 without altering the over-all image size at 5. Proper adjustment of the lens 18 will thereby provide a real, inverted image of unity magnification which is in focus at least in the annular regions thereof. This demonstrates that the individual bundles may be focused without affecting image magnification. Examining the geometry of the total beam 16 shown in FIG. 3, between the plane of the photocathode 1 and the lens 15, it follows a generally collimated path; after the lens 15 it converges to the cross-over 17 following which it diverges. In this diverging condition, it intersects the image surface 5. This condition causes non-linear, pincushion geometric distortion in the outer peripheral regions of the image which are corrected by introduction of a third lens 19 along with the other two lenses 15 and 18 as shown in FIG. 4. In this figure, like numerals indicate like parts. The lens 19 is preferably of the same power as the lens 15 such that the total beam 16 as it emerges from the lens 19 is in essentially the same collimated form as it is on the input side of the lens 15. In the plot of FIG. 4, that portion of the beam 16, indicated by the numeral 20, on the left-hand side of the cross-over 17 is a fold-over duplicate of that portion 21 of the beam 16 on the right-hand side thereof. The image thereby focused in the plane 5 is now a faithful reproduction of that emitted by the photocathode 1 and is self-corrected for the non-linear pin-cushion distortion briefly alluded to hereinabove.

Geometric distortion is reduced, because over-convergence in the outer regions of lens 15 in FIG. 4 due to spherical aberration are compensated in the outer regions of lens 19. By using unity magnification in addition to collimating the beam between lens 19 and plane 5, bilateral symmetry of the beam geometry about the cross-over 17 is achieved which self-corrects for distortion and provides for maximum resolution.

Now referring to FIG. 5, a practical phototube design which provides the electrostatic lens configuration of FIG. 4 will be described. An evacuated glass envelope 26 is provided with two substantially flat end faces 27 and 28 transparent to the desired radiation. On the inner surface of the end face 27 is deposited the photocathode 1. 0n the inner surface of the end face 28 there is deposited a phosphor anode 29 corresponding to the previously discussed image surface 5. A drift tube 30 of metal or similar conductive material is mounted rigidly within the envelope 26 by means of suitable insulators or the like (not shown). The drift tube 30 is positioned such that the axis thereof is normal to the centers of the end faces 27 and 28. A flat fine mesh screen 31 mounted on a supporting ring 32 is also rigidly mounted within the envelope 26 in spaced relation between the cathode 1 and the left-hand end 33 of the drift tube. The screen 31 as well as its supporting ring 32 are electrically insulated from the drift tube 30. The lens 15 of FIG. 4 is provided by the left end 33 of the drift tube and the screen 31, this lens configuration being conventional and normally referred to as a bi-potential lens. Another similar screen 34 mounted on a supporting ring 35 is rigidly mounted inside envelope 26 in spaced parallel relation between phosphor screen 29 and the right-hand end 36 of drift tube 30. This right-hand end 36 and the screen 34 provide the bi-potential lens indicated by numeral 19 in FIG. 4.

Midway between the ends of drift tube 30 is mounted an Einzel lens generally indicated by numeral 18, this lens including two conductive discs 37 and 38 spaced apart in parallelism and conductively secured at the peripheries thereof to the drift tube 30 as shown. These discs 37 and 38 are provided with central apertures 39 and 40, respectively, concentric with respect to the axis of the drift tube 30. A third disc 41 of metal or similar conductive material is coaxially positioned inside drift tube 30 between the two discs 37 and 38 in parallelism therewith. This disc 41 is spaced equal distances from the two discs 37 and 38 and in turn is mounted within drift tube 31 so as to be insulated electrically therefrom. Glass supporting elements (not shown) securing the perimeter of disc 41 to the inner surface of drift tube 30 are preferably used.

Disc 41 is provided with a central aperture 42 concentrically surrounding the axis of drift tube 30, this aperture 41 being smaller in size than the two apertures 39 and 40. These latter apertures preferably are of equal size.

This Einzel lens is disposed at the cross-over point 17 of the beam 16. Further information regarding the design and construction of a suitable Einzel lens may be found in previously filed application Ser. No. 597,618 filed Nov. 29, 1966, Case #12, entitled Camera Tube Having 8. Variable Resolving Aperture, of the same inventor.

Potentials are applied to the various electrodes just described by means of the circuitry shown, these potentials being adjusted and being supplied by, for example, a battery 43. These potentials along with the sizes and spacings of the various electrodes are adjusted such as to provide the lens configurations described in connection with FIG. 4. The power of the two lenses 1S and 19 are adjusted to be the same. Thus, the potential ratios between the respective screens 31 and 34 and the adjacent ends of the drift tube 30 are made the same. The power of the lens 18 is chosen such that bundle convergence occurs beyond the lens 19. It has been found that a power of lens 18 approximately twice that of one of the lenses 15 or 19 is suitable. Adjustment of the power of the lens 18 may be effected by adjusting the potential difference between the disc 41 and the two discs 37 and 38. In the illustrated example, the disc 41 is fifty (50) volts higher in potential than the cathode; however, it will be obvious to persons skilled in the art that this voltage may be altered to obtain the proper convergence. In the adjustment, the voltages on the Einzel lens 18 are changed until the necessary reconvergence is achieved, this being observed visually on the phosphor screen 29.

In operation, assuming that a radiation image is optically projected onto the end face 27, the cathode 1, being composed of a layer or coating of photoelectric material, emits electrons in response thereto. These electrons are accelerated by the electrode structure 30 and 31 and further are formed into a beam which, like the beam 16 of FIG. 4, converges to a cross-over inside the aperture 42 and thereafter diverges until reaching the space between the tube end 36 and screen 34. The electrostatic field or the lens effect produced by the potential difference between the screen 34 and the tube end 36 exerts a convergent force on the diverging beam 16, and in an ideal tube design causes the distal portion of the beam to have the same collimated shape and follow essentially the same path, but in reverse, of the initial portion of the beam 16 flowing between cathode 1 and screen 31.

Inasmuch as the Einzel lens 18 (FIG. 5) is situated precisely at the beam cross-over, it will act on the electrons tending to follow divergent paths and cause them to converge as already explained in connection with the preceding figures. The potential on disc 42 may be adjusted to control the rate of convergence.

In this device of FIG. 5, since unity magnification is used, the entire area of cathode 1 can be utilized for developing an electron image which is focused on substantially the same size area of phosphor screen 29. By using such a weak lens, spherical aberration is kept to a minimum. Also, since unity magnification produces only a small divergence angle 0 of the beam, distortion found in wider beam angles wherein higher ratios of magnification are used is avoided. By using the lens configuration 19 whereby the condition of bilateral symmetry is obtained, geometric distortions produced by the initial lens are compensated. These conditions of minimal distortion are attributable in major part to the fact that the electrodes and electrostatic fields developed thereby are bilaterally symmetrical about a plane perpendicular to the tube axis at the point 17 of the cross-over. By matching electron transit times between cathode 1 and screen 31 on the one hand the phosphor anode 29 and screen 34 on the other, the condition of bilateral symmetry is obtained. Under these conditions, a plane-parallel field between the respective screens 31 and 34 and the adjacent cathode 1 and anode 29 prevails.

The material of cathode 1 is conventional and corresponds to that used in television camera tubes or image tubes wherein it is desired to develop electron images corresponding to incident radiation images. The phosphor composing screen 29 is conventional and may be the same as that used in image converters or on the display faces of ordinary television-receiving tubes.

What is claimed is:

1. A low distortion phototube comprising an element capable of emitting an electron image, means for forming said electron image into a beam and focusing said image in inverted form into a plane spaced a predetermined distance from said element, an electron-collecting device located in said plane for providing a representation corresponding to said inverted image; said focusing means including first lens means adjacent to said element for converging said beam to a cross-over point disposed between said element and said device, said beam thereafter diverging from said cross-over point, second lens means adjacent to said cross-over point for reconverging electrons of said beam having initial divergent trajectories upon emission from said element, and third lens means adjacent to said device for exerting a converging force which corrects distortion in the diverging peripheral por tion of said beam.

2. The phototube of claim 1 in which said first, second and third means includes electrode structure for developing electrostatic fields between said element and said device which are bilaterally symmetrical about a plane located at said cross-over point perpendicular to said beam axis.

3. The phototube of claim 1 wherein said element is an extended area photoelectric cathode and has an electronemitting surface portion substantially normal to said beam axis.

4. The phototube of claim 3 in which said first and third lens means are bi-potential lenses.

5. The phototube of claim 4 in which said second lens means is an Einzel lens.

6. The phototube of claim 5 in which said device is a phosphor screen of extended area.

7. The phototube of claim 3 wherein said first and third lens means includes a conductive elongated drift tube concentric about said axis and having opposite ends disposed adjacent to said cathode surface and said device, respectively, a first conductive screen electrode disposed between and being insulated from said cathode surface and one of said tube ends, said screen extending substantially parallel to said surface, a second conductive screen electrode disposed between and being insulated from the other end of said drift tube and said device, said second screen having an extended area portion substantially normal to said axis.

8. The phototube of claim 7 in which said second lens means includes an Einzel lens structure disposed inside said drift tube between the ends thereof.

9. The phototube of claim 8 in which said device is a phosphor screen of extended area substantially parallel to said second conductive screen.

10. The phototube of claim 9 further comprising an evacuated envelope mounting spaced apart transparent faceplates, said cathode being on the inner surface of one faceplate, said phosphor screen being on the inner surface of the other faceplace, said cathode surface and said phosphor screen being substantially flat and parallel.

11. The phototube of claim 10 in which said Einzel lens comprises two adjacent parallel discs normal to said axis having central apertures concentric about said axis, said discs being conductively connected to said drift tube, a third disc disposed between and in parallelism with said two discs, said third disc being insulated from said drift tube and having an aperture concentric with respect to said axis.

12. The phototube of claim 9 including means for equalizing the transit time of electron flow between said cathode and said first conductive screen and said phosphor screen and said second conductive screen.

13. A low distortion phototube comprising an elongated evacuated envelope having opposite ends, said ends being provided with transparent faceplates, respectively, extending substantially normal to an axis within said en velope, a conductive drift tube mounted in said envelope and coaxially surrounding said axis, said drift tube having opposite ends which define planes substantially parallel to said faceplates, respectively, a photoelectric cathode of extended area on the inner surface of one faceplate opposite one drift tube end, a phosphor screen on the inner surface of the other faceplate opposite the other drift tube end, a first extended area conductive screen interposed between said cathode and said one drift tube end, said first screen extending substantially normal to said axis and forming a positive lens element with said one drift tube end, a second extended area conductive screen interposed between said phosphor screen and said other drift tube end, said second screen extending substantially normal to said axis and forming a positive lens element with said other drift tube end, and an Einzel lens within said drift tube at a predetermined location between the ends thereof, said Einzel lens having a beam-transmitting aperture coaxially surrounding said axis whereby an electron beam may be transmitted therethrough in transit from said cathode to said phosphor screen.

14. The phototube of claim 13 in which the sizes of said cathode and phosphor screen are substantially equal,

8 said Einzel lens is disposed midway between the opposite ends of said drift tube.

15. The phototube of claim 13 and including first means applying a potential between said first screen and said drift tube for converging an electron beam issuing from said cathode to a cross-over at said Einzel lens aperture, said second means applying a potential between said second screen and said phosphor screen for collimating said electron beam after it passes through said aperture, and third means for applying potentials to said cathode and said phosphor screen for accelerating electrons first from said cathode to said first screen and secondly from said second screen to said phosphor screen.

References Cited UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner.

C. LEEDOM, Assistant Examiner.

U.S. Cl. X.R. 

